The invention described herein is a result of work performed at Lawrence Livermore Laboratory under Contract W-7405-ENG-48 between the United States Department of Energy and the University of California.
The invention relates to methods of backfilling abandoned in-situ retorts using spent oil shale, and more particularly to a method of producing grout by treating burned shale to maximize the production of hydraulic compounds from carbonate minerals.
Vast amounts of oil shale rock are found in the western United States, particularly Colorado. Oil shale contains a polymeric organic material, kerogen, which decomposes upon heating to give oil. The oil reserves present in the oil shale in the United States are estimated to be greater than all the oil reserves in the Middle East.
One method of producing oil from shale is a modified in-situ retorting process in which rubblized oil shale is burned underground to yield oil and the oil is pumped out. A retort, with lateral dimensions up to 100 m.times.100 m and height up to 300 m, is ignited at the top and a combustion zone is forced downward by flow of oxygen-containing gas. Oil is released ahead of (below) the combustion zone. However, the in-situ retorting process cannot occur unless the underground material is first made permeable by creating void space in the retort. Accordingly, approximately 20% of the material in the region to be processed is removed by conventional mining techniques. The remaining material is rubblized to make it permeable to permit in-situ retorting. The material removed is retorted on the surface to produce additional oil. Three basic types of surface retorts are known in the art, the combustion retort, the hot gas retort, and the hot solid retort.
There are several severe environmental drawbacks to the in-situ retorting process. The spent shale left after retorting may compress, causing subsidence of the surface. The materials remaining in the retort after processing can also release compounds into flowing ground water. Toxic gases may be released. The surface retorting of the material removed produces waste which also must be disposed.
A solution to these problems is to put the waste material produced by the surface retorting back underground, filling and solidifying the in-situ retort. However, since the mining tunnels used for the removal of the material have been destroyed in the rubblization process, the material cannot be replaced as a solid. It has been proposed to convert the material into a slurry for injection into the retort.
Several United States patents are directed to methods of treating spent oil shale, including methods of producing a slurry for backfilling an in-situ retort.
U.S. Pat. No. 4,096,912 to Lewis and Mallon, issued June 27, 1978, shows a method of minimizing plastic flow of hot rubblized oil shale in an in-situ retort by introducing carbon dioxide and water into the spent shale above a downwardly moving retorting zone. The reaction with the mineral consituents of the spent shale forms a cement-like material. It is preferable to first burn the spent shale in-situ to remove carbonaceous residue. The water may be introduced as steam.
U.S. Pat. No. 4,131,416 to Watson and Spear, issued Dec. 26, 1978, describes the heating of spent oil shale to temperatures ranging from 1200.degree. to 1500.degree. F. and mixing the treated shale with water to form a slurry for backfilling a in-situ retort. An additional slurry of sand or other porous material is also added to the retort to allow drainage of the water.
U.S. Pat. No. 4,120,355 to Knepper and Grossman, issued Oct. 17, 1978, also describes the treatment of spent oil shale at 1200.degree. to 1500.degree. F. for the production of backfilling slurry.
U.S. Pat. No. 3,459,003 to O'Neal teaches adding calcium carbonate and calcium oxide, clay or gypsum to spent oil shale and heating the mixture to form cement clinker. This cement clinker may then be mixed with water and reintroduced to the mined-out area as a slurry.
U.S. Pat. No. 3,135,618 to Friese, issued June 2, 1964, discloses a process for producing cement using pretreated oil shale. The oil shale is pretreated by fluidized bed partial oxidation at 400.degree. to 650.degree. C. in the presence of a reduced oxygen supply. The pretreated oil shale is then mixed with calcareous material and heated to high temperature for the production of cement.
U.S. Pat. No. 2,434,815 to Shaw, issued Jan. 20, 1948, describes a method for separating oil from oil shale by contacting the shale with super-heated steam at a temperature between about 600.degree. and 1200.degree. F. Steam is brought into contact with hot spent shale to produce super-heated steam.
The available methods for treating spent shale to produce a slurry result in a slurry with a high water content which does not solidify easily and therefore cannot support the overburden or stop water flow. Therefore, the environmental problems of subsidence and leaching are not eliminated. Furthermore, the process requires large amounts of water which is a very scarce resource in the parts of the United States where oil shale deposits are found and there are many high priority competing uses such as agriculture for the available water supply. The failure of the slurry injection methods is described in the study of the Denver Research Institute May 1977 Report.
Accordingly, it is an object of the invention to provide a method for stabilizing abandoned in-situ oil shale retorts.
It is a further object of the invention to provide a method for treating surface processed shale to produce a grout for stabilizing abandoned in-situ oil shale retorts.
It is a further object of the invention to produce a grout from spent shale which is sufficiently non-viscous for injecting into a retort and becomes rigid and strong upon setting.
It is also an object of the invention to provide a process for reacting carbonate minerals in oil shale.
It is another object of the invention to provide a process for treating spent shale which maximizes the conversion of carbonate minerals into hydraulic compounds.
It is a further object of the invention to produce a grout with a maximum composition of the compounds alite and larnite.